Inorganic insulating coating for electrical steel sheet and strip



Patented June 7, 1949 George 13. Kiefer, Springdale, =:Ba., v.assignor to "Alleghenylmdlumsteel Gorpotation, acorpora- 'tionof Pennsylvania.

NoDrawing. Application ianuary il, 1945, Serial No. 512x97 v Claims. I This invention relates to the production of a new and improved coatingfforsteel'istrip orsheet material such as is employed inthe make-up of electrical apparatus.

An object of the invention is toprovide a coating material which "is capable of being spread uniformly over strip orsheet metal, such 'for example as silicon steel, and"which,when baked, acquires excellent insulating properties but is of such character as not to :detrimentally effect the tools and dies employed in fabricatingv such strip or sheet material.

A further object is 'toiprocluce an insulating coating .for electrical sheet and :strip material which is capable cfbeing'sprea'd infilm-like' form and substantially uniformly over the-surface of the sheet orstrip'to be coated and-which, in the finished state, has high insulatingproper-ties, is insoluble in water and transformer oil and is not detrimentally affected when subjected to temperatures such as areeneountered in the use of electrical sheet or strip-material 'orin 'the'preparation of such material for use.

A further object is to produce a coating material for strip or sheet "steel WhichyWhenapplied and baked, provides'a protective coating for the covered surface and improves the punching characteristics of'the steelsocoated.

These and other obj'ects'areattained'by means of a coating material'embodying the'characteristics herein set forth andformed-fromconstituents herein set forth or substantiahequivalents of the same.

The coating material as formed'consists of a mixture of magnesium chromate (MgCrQzlf magnesium acetate (MgfllzHaOzh) and water (H2O) The preferredmixtureincludes:

397 pounds magnesiumchromate,--7 i-I20 (equivalent of 209.2 pounds'MgCrOi) 251 pounds magnesium acetate,..4 H2O (equivalent of 156.? pounds of MgiCzHsOzh), and suificient water-to make lfiflgallonsrdfisolution.

I have also prepared the coatingby mixing 97.1 pounds of magnesium oxide '(MgO), 154 pounds of chromium trioxide (chromic acid CrOa) 126 pounds of glacial acetic' acid CHQCQOH and sufficient water to make liltkgallons of=solution.

Solutions of magnesium .chrcmate "(or "ma nesium oxide and chromicacid) without'the-addition of magnesium acetate tor' acetic acidand magnesium oxide) will'produce acoating. mikewise, solutions of magnesium acetate (or acetic acid and magnesiumoxide) without-the-addition of mmag-nesium chromate (or magnesium oxide and chromic @acid) also produces :a coating. These coatings, :however, are difficult to bake, tend to flake off of the carrying metal after bakingxand are not satisfactory functionally.

. Ih'ave alsofound that, inorder to obtainrasgoo'tl adherence :ofthe coatmg'to'the -metal smface, a proper: ratio of chromi-c acidto acetic 'acid acr magnesium chromate to magnesiumacetate'must be .adheredlto. For example, whenlthe wei'glit-iif magnesium chromate exceeds the weightbflnflgfnesium'iacetate by more than two to one, the-resultant coating tends to flake ofi after" baking. "On the other hand, when the weight of thema'g rnesium 'chromate is less than that 'of' themagrnesium acetate, the resultant coating is illscolored and is slightly brittle when baked. l-he amount of magnesium 'chromate by weight should, therefore, exceed that of the magnesium acetate but be less than twice the-weightcf the acetate.

After the aqueous solution, as above defined, is applie'd to thesurface to-be coated, it is"baked Tor 'a "sufiicient time to 'dehydrate and also de- -:compose-=the*salts of the solution. The baking temperature maybe-as low-as 400 F. or ashi '-as- 1750*F. I,- however, prefer toemploy a-paking temperature of about 850 because a'temperature ineXCBSS of that tends to evaporate the water in the mixturetoo rapidlyand to'o'ccasion 'blistering an'd "subsequent flaking. The "time -pf baking will vary with the thickness of-the coated metal. Baking at 800 'F. for'two'minutes' (total exposure), has disclosed "itself as suflicient "in connection "with coated sheets of "0.014 thick- "ness. Thinner sheets will require less time to bake the "coated'material, and thicker sheets will require more time.

--While the above disclosure defines the preferred solution "and "the preferred Procedure "for making "the same, "a suitable coating material can "be 'made by the above enumerated constituents within'the limits defined as follows:

85.7.5 to121.5:poundsmagnesium oxide (MgO) 124.5- to -l pounds chromium *trlcxide- (chromic acid) CrOs to 148 pounds glacial acetic-acid ((3213462),

' iandzsumcient water to make 1'00 gallons; *0!

330 to 473 pounds mag-nesimn 'chromate, T-Hi0 (equivalent of 1751:0250 pounds Mg'CrOO I '18! to 263 "pounds magnesium/acetate, 4 iHi'O (equivalent of 'to1'75pounds Mg ('CzHs'Ozlt) and sufilcient water to make 100 gallons.

These solutions applied on metal surfaces and baked, as defined herein, result in coatings which are insoluble in water or oil, possess excellent insulating properties and are not destroyed or detrimentally affected when subjected to temperatures as high as 1750 F. As a matter of fact, coatings subjected to such temperatures for an appreciable period (about 30 minutes) have disclosed increased insulating values.

While the constituents of the coating solution are soluble in water as above disclosed, the baking temperature not only dehydrates the solution defined but it also decomposes the constituents thereof and the resulting coating is insoluble in water and oil. The coating, while of complex chemical composition, undoubtedly includes magnesium oxide, chromium oxide and some iron oxide. The iron oxide possibly results from the fact that some oxygen is released from the magnesium salts during their decomposition and, while in a more or less nascent state, forms an oxide with the metal to which the coating is applied with the result that such metal also provides an oxide or other constituent of the coating.

The procedure herein set forth, as an embodiment of my invention, may be briefly stated as coating electrical strip or sheet material with a thin film of an aqueous solution containing magnesium chromate and magnesium acetate (no matter how formed) and in which the amount of chromate by weight is preferably greater than that of the acetate but less than twice the weight of the acetate, and then subjecting the material so coated to sufficient heat to dehydrate such film and to decompose the magnesium compounds constituting constituents thereof. The baked or dehydrated coating not only provides an insulating film which is highly effective when applied to electrical sheet or strip material used in the fabrication of electrical apparatus but, it also provides a protective coating which facilitates the punching or shearing of metallic sheet or strip material whether or not the same is formed from electrical sheet. It, therefore, is apparent that the dehydrated coating does not have a detrimental efiect on the tools and dies employed in the fabrication of such material. I have also found that the dehydrated coating gives good results as a protective coating for steel sheet or strip material that has either been parkerized, bonderized, or treated with some similar phosphate coating.

While coatings embodying my invention may be varied as to their constituents, within the limits above defined, all such coatings, nevertheless, involve mixtures of inorganic materials such as defined and produces a coating for electrical sheet and strip material which has many times the resistance value of other commercial coatings now employed for such materials. It will be understood by those skilled in the art that some additional materials may be addedas constituents to coatings, such as are above defined, without departing from the spirit and scope of my invention as defined by the appended claims and that the temperature range for baking the same may be widely varied within the limits above defined.

What I claim is:

1. A method of forming an insulating coating on electrical steel in sheet or strip form, which consists in spreading an aqueous solution over the surface of such steel to form a film-like coating thereon, which solution consists essentially of magnesium chromate and magnesium acetate in which the chromate exceeds the acetate by weight in an amount less than twice the weight thereof, then in subjecting the coated steel to sufiicient heat at a temperature of about 850 F. to dehydrate such coating and decompose the magnesium salts contained therein.

2. A method of forming an insulating coating on silicon steel in sheet or strip form, which consists in coating the surface of such steel with an aqueous solution the solutes therein consisting essentially of magnesium chromate and magnesium acetate in the proportions of about two hundred parts by weight of chromate and one hundred and fifty parts by weight of acetate and then subjecting the coated steel to sufficient heat within the range of from about 400 F. to 1750 F. to dehydrate such film and decompose such salts.

3. A method of forming an insulating coating on silicon steel in strip or sheet form, which con-- sists in coating at least a surface of such steel with an aqueous solution the solutes therein consisting essentially of magnesium chromate and magnesium acetate in which the chromate exceeds the acetate by weight but in an amount less than twice the weight thereof, then subjecting the material so coated to sufiicient heat within the temperature range of from 400 F. to about 1750 F. to dehydrate such coating and to decompose the magnesium salts contained therein.

4. A method of forming an insulating coating on silicon steel in sheet or strip form, which consists in coating the surface of such steel with an aqueous solution of magnesium chromate (71-120) and magnesium acetate (4H2O) in the proportion of from about 330 to about 473 pounds of magnesium chromate and from about 187 to about 263 pounds of magnesium acetate in approximately sufficient water to make gallons of solution with the amount of chromate in excess of the acetate by weight but in an amount less than twice the weight thereof, then subjecting the material so coated to heat within the range of from about 400 F. to 1750 F. for a sufiicient period to dehydrate such coating and decompose said magnesium salts.

5. An electrical steel sheet coated with a firmly adherent coating insoluble in oil and water and including as essential constituents thereof magnesium oxide and chromium oxide, resulting from the dehydration and decomposition by heat within the range of from 400 F. to 1750 F., of an aqueous solution while spread over the surface of such sheet, the solutes thereof consisting essentially of magnesium chromate and magnesium acetate in which the chromate exceeds the acetate by weight but in an amount of less than twice the weight of the acetate.

GEORGE C. KIEFER.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 589,221 Placet Aug. 31, 1397 2,144,425 Cook Jan. 17, 1939 2,210,850 Curtln Aug. 6, 1940 2,224,245 Allen Dec. 10, 1940 2,308,070 Frey Jan. 12, 1943 2,354,123 Horstman et a1 July 18, 1944 2,385,332 Carpenter et a1 Sept. 25, 1945 Certificate of Correction Patent No. 2,472,592. June 7, 1949.

GEORGE o. KIEFER It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 4, lines 39 and 40, claim 4, strike out the Word approximately and insert the same after make in line 40;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent OfiEice.

Signed and sealed this 1st day of November, A. D. 1949.

THOMAS F. MURPHY,

Assistant Gammz'ssz'oner of Patents. 

